Many operations require a continuous supply of fluid, usually water, from a large fluid source such as a reservoir or river. Often water is pumped in order to provide a cooling fluid for critical heat transfer devices such as steam condensers in power plant operations where the loss of the continuous supply of cooling water can have disastrous consequences on the continued operation of equipment. Thus it is important to have a continuous reliable supply of cooling fluid.
In order to supply large volumes of fluid, suction pumps are usually employed in the natural environment of a reservoir or other large source of fluid. Suction pumps are suspended in an intake structure usually made of steel and concrete in the form of a large enclosed chamber which supports and protects the suction pump. Commonly one end of the enclosed chamber is open to the fluid source except for "grizzley bars" over the submerged opening in the intake structure and which serve to rough screen out large debris, large fish and other water creatures that may be attracted to the intake. Intake water which travels through the grizzley bars into the enclosure is usually subjected to a finer mesh screen of a fixed or continuously rotating nature which serves to further protect the suction pumps and downstream equipment from finer debris particles and small fish. Even after passing through these screening devices some floating debris or chunks of ice can still be present in the dead ended portion of the intake structure closest toward the land where the suction pump is usually located.
Most of the water sources that are available for such uses are subject to natural variations in elevation caused by such things as tides, ice jams, rainfall, evaporation, or the operation of hydroelectric dams in waterways.
The suction pumps employed are usually vertical centrifugal pumps which extend through the top of the intake support structure into the pumping chamber while remaining a sufficient distance above the bottom of the pumping chamber so as to avoid restrictions on permissible flow into the pump intake and impeller. There are many different manufacturers and impeller designs for suction pumps but each individual suction pump has its own pump characteristics which are fixed. These are well known and are plotted on graphs which relate the flow rate of the pump to the pump submergence depth in the pumping fluid. Once a particular intake support structure is fixed in place, the pump chamber water elevation can be used in these plots as a measure of submergence depth.
There are two main problems that arise in the operation of suction pumps if the fluid level in the pumping chamber falls below the minimum submergence depth required for any particular pump. The first of these is called vortexing and the second may be referred to as cavitation. This invention is directed toward elimination of the vortexing component of the submergence requirement for a given pump so that it may be operated where the natural level of the fluid source provides less than the minimum required submergence depth for vortexing. This situation usually occurs when a large and expensive fixed pump intake structure is in place and the water level in the reservoir drops below the expected minimum level or when because of the difficulty of calculating the resistances in pipes and components connected to the output of the pump, the flow rate varies from designed flow rate which in turn moves the pump operation to a different place on the characteristic curve to where a greater than original design submergence would be required for proper operation.
Vortexing is a condition which begins once the pump is operated below the minimum required submergence depth and gradually worsens as submergence depth is decreased until finally it allows air to enter the pump through a "tornado" shaped cone which forms at the surface of the fluid. The vortices may be multiple in nature. In addition to undesirably entraining air into the pump it creates hydraulic disturbance in the flow pattern of the fluid and results in unwanted mechanical forces on the pump which may cause damage. Entrained air seriously interferes with upstream heat transfer equipment and generally causes problems in closed systems. The swirling vortices entrain floating debris down through the fluid to the pump intake to further enhance the pump disturbances and they provide material to clog and otherwise interfere with upstream equipment. Pump efficiency can be adversely affected by these conditions.
Usually vortexing is the critical limitation on submergence depth and once this limitation is removed the pump can be operated at a reduced submergence level until it reaches the reduced depth where cavitation becomes a problem. Cavitation is the formation of water vapor bubbles and a subsequent sudden collapse of the bubbles which causes pump noise, rough operation of the pump, and damage under prolonged operation in that condition. The cavitation condition is often represented by NPSHR which refers to net positive suction head requirement. NPSHA is a term used which means the net positive suction head available. Curves similar to the vortexing curves are drawn for pumps relating the NPSHR in terms of flow rate and submergence depth of a pump. They are characteristic curves just like the vortexing curves and the failure to maintain the required NPSH give rise to the pumping and impeller wear problems which are generally recognized under the label cavitation. Once the vortexing problem is removed then the minimum submergence depth is controlled by the NPSH curve characteristic of the pump which as was indicated is usually much lower than the minimum depth required by the vortexing characteristic of the pump. Previous attempts to solve the vortexing problem when the submergence level is less than what the characteristic curve requires are basically devices that interfere with the formation of vortexes in the pump fluid. They generally take the form of a floating raft usually in the form of a grid or a perforated steel plate or a screen which is located at or near the surface of the fluid level in the pumping chamber. These devices may be expensive to build and difficult to maintain and present the potential problem of pluggage with debris or ice formations which can cause even more serious problems with pump operation. They can become plugged in a relative short time and it is difficult to predict when plugging will occur. The condition of the screen or plate is difficult or impossible to monitor and maintain and it is difficult to correct the problem when it does occur without significant and disruptive down time of the pump.